1. Field of the Invention
The present invention relates to particles for catalyst which have a function of absorbing and desorbing oxygen, and a method for producing the particles for catalyst. For example, the particles for catalyst can be used as a promoter (co-catalyst) of an automobile exhaust gas purification catalyst.
2. Description of the Related Art
As an automobile catalyst for removing hazardous components such as HC, CO and NOx, from an automobile exhaust gas, for example, noble metals such as Pt, Pd and Rh are used. These noble metals are supported on the surface of a carrier such as alumina so as to increase a contact area with the exhaust gas, and thus various hazardous components are purified.
In the automobile catalyst, it is necessary to control an air-fuel ratio (A/F), which varies depending on engine operation conditions, in a fixed narrow range so as to increase a purification rate. Therefore, an oxygen sensor maintains the A/F and provides optimum combustion conditions and reaction conditions for purification of an exhaust gas.
However, a trace amount of a hazardous gas contained in the exhaust gas is actually purified by adsorbing on a catalytic noble metal or a promoter on a carrier and causing the catalytic reaction therewith. Proceeding a series of these reaction processes requires high purification performances because reaction conditions, particularly gas composition, have to be maintained in considerably micro-sized small space.
In a current engine system, it is impossible to sufficiently exhibit purification performances of the catalyst only by a macro-controlled A/F value. Therefore, a function capable of controlling the A/F value in the micro-sized space is required to the catalyst itself. It is an action referred to as an oxygen storage ability (OSC capacity) of the automobile catalyst, which attain such a function, and a promoter (co-catalyst) plays the role.
The particles for catalyst are used as the promoter, and ceria or a ceria-zirconia solid solution, having a function capable of absorbing or desorbing oxygen, namely, an OSC capacity is generally employed (see, for example, Japanese Unexamined Patent Publication (Kokai) Nos. 2001-198461, 2000-153160 and 2004-136179).